Electron transport chain, chemiosmosis and oxygen

Molecules (Interaction and interdependence) · Cell respiration · note 7 of 8

Spec C1.2.13, C1.2.14, C1.2.15, C1.2.16

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Electron transport chain, chemiosmosis and oxygenSpec C1.2.13, C1.2.14, C1.2.15, C1.2.16

In short

Reduced NAD from glycolysis, the link reaction and the Krebs cycle passes a pair of electrons to the first carrier of the electron transport chain on the inner mitochondrial membrane. Electron flow releases energy that pumps protons into the intermembrane space. Protons diffuse back through ATP synthase, which phosphorylates ADP: chemiosmosis. Oxygen is the terminal electron acceptor, forming water.

Reduced NAD delivers energy to the chain

Reduced NAD comes from glycolysis, the link reaction and the Krebs cycle. Energy is transferred when reduced NAD passes a pair of electrons to the first carrier in the electron transport chain, a series of carriers embedded in the inner mitochondrial membrane (on the cristae). This converts reduced NAD back to NAD, which can return to accept more hydrogen.

A proton gradient

The electrons pass from carrier to carrier. At each step they lose energy, and some carriers use this energy to pump protons (H⁺) from the matrix across the inner membrane into the intermembrane space. Because the inner membrane is impermeable to protons, a proton gradient builds up: a higher concentration of protons in the intermembrane space than in the matrix.

Chemiosmosis and ATP synthase

Chemiosmosis is the movement of protons down their concentration gradient through ATP synthase. Protons can only diffuse back into the matrix through channels in ATP synthase. As they flow through, part of the enzyme rotates, and ATP synthase couples the release of energy from the proton gradient with the phosphorylation of ADP, forming ATP. Most of the ATP from aerobic respiration is made this way.

Oxygen as terminal electron acceptor

At the end of the chain, oxygen accepts electrons from the last carrier and protons from the matrix, producing metabolic water.

½O₂ + 2e⁻ + 2H⁺ → H₂O

Removing electrons at the end allows continued flow of electrons along the chain. Without oxygen the carriers stay reduced, reduced NAD cannot be oxidised, the proton gradient is not maintained and no NAD is available for the link reaction and Krebs cycle, so aerobic respiration stops.

Cross-section of the inner mitochondrial membrane: reduced NAD gives two electrons to the first of three electron carriers; electrons pass along the chain while each carrier pumps H⁺ from the matrix into the intermembrane space; oxygen accepts electrons and H⁺ to form water; H⁺ flows back through ATP synthase into the matrix (chemiosmosis), forming ATP from ADP and Pi. — Electron flow pumps protons into the intermembrane space; they return through ATP synthase (chemiosmosis). Oxygen is the terminal electron acceptor.

Exam tip:

Give the direction precisely: protons are pumped into the intermembrane space and flow back into the matrix through ATP synthase. You do not need the names of the protein complexes.

Written and checked against the IB Biology HL specification · Updated October 2026

Frequently asked questions

Why is ATP called the energy currency of the cell?

ATP is called the energy currency because it distributes energy within cells. It is soluble, stable without enzymes and stays inside the cell, and hydrolysis to ADP and phosphate releases enough energy for tasks such as active transport, building macromolecules and movement. It is quickly regenerated from ADP using energy from respiration.

What is the difference between cell respiration and gas exchange?

Cell respiration is a set of enzyme-catalysed reactions inside cells that produce ATP using energy from carbon compounds. Gas exchange is the diffusion of oxygen and carbon dioxide between an organism and its environment, for example in the lungs. Gas exchange supplies oxygen for aerobic respiration and removes the carbon dioxide it produces.

What are the differences between aerobic and anaerobic respiration?

Aerobic respiration needs oxygen and mitochondria, can use carbohydrates, lipids and amino acids, gives a large yield of ATP and produces carbon dioxide and water. Anaerobic respiration in humans needs no oxygen, happens only in the cytoplasm, uses only glucose, gives just 2 ATP per glucose and produces lactate.

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